A basic tone control is a capacitor and a variable resister, Can't see that increasing the cost of the battery use much, if at all.
I expect it's more because they want to create the impression that DAB is a purer sound and doesn't need equalisation at all (this is purely my opinion) whereas in reality DAB radio is actually lower quality than FM radio (to get an equivalent quality to FM DAB would need to be broadcast at about 196KBS and most DAB stations are broadcast in 128KBS or lower)